1. Field of the Invention
The present invention relates to a mobile communication system and, particularly, to a mobile communication system for automobile phones, cellular phones, or PHSs (personal handy phones), capable of registering, deleting the registration of, and calling the phones, i.e., mobile stations in narrow paging areas controlled by the system.
2. Description of the Related Art
FIG. 15 shows a mobile communication system according to a prior art. The system includes a location management center 15-1, exchange stations 15-2, location areas 15-3, base stations 15-4, and mobile stations. Location area identifiers LAI1 and LAI2 are assigned to the location areas 15-3, respectively.
To identify the location of a mobile station 15-5 in the system, the service area of the system is divided into the location areas 15-3. Each location area 15-3 contains the base stations 15-4 each of which periodically transmits a radio signal containing a location area identifier such as LAI1 and LAI2 assigned to the location area to which the base station belongs.
The mobile station 15-5 receives the radio signal and stores the location area identifier therein. When moving from the location area LAI1 to the location area LAI2, the mobile station 15-5 detects that the location area identifier LAI1 presently held therein differs from the newly received location area identifier LAI2, and transmits a location registration request to the base station 15-4.
In response to the location registration request, the base station 15-4 sends a location registration request to the center 15-1 through the exchange station 15-2. The center 15-1 registers the location area identifier LAI2 for the mobile station 15-5.
If an incoming call to the mobile station 15-5 occurs, the exchange station 15-2 inquires of the center 15-1 a location area where the mobile station 15-5 is present, acquires the location area, and transmits a paging request to all base stations 15-4 belonging to the acquired location area. In response to the paging request, these base stations 15-4 simultaneously page the mobile station 15-5 in the respective radio zones.
In this way, the mobile station 15-5 transmits a location registration request whenever it moves from one location area to another. If an incoming call to the mobile station 15-5 occurs, all base stations 15-4 in the location area where the mobile station 15-5 is present simultaneously page the mobile station 15-5. If each location area 15-3 is large, many base stations 15-4 therein must simultaneously page a mobile station whenever an incoming call arises to the mobile station. Namely, many base stations 15-4 whose radio zones do not involve the mobile station must make useless paging to deteriorate the effective use of radio resources.
If each location area 15-3 is small, the mobile stations must frequently issue a location registration request when moving from one location area to another. This congests network traffic and puts heavy load on the center 15-1.
An object of the present invention is to provide a mobile communication system capable of reducing the size of paging areas to reduce useless paging and improve the effective use of radio resources without increasing location registration requests to a location management center.
In order to accomplish the object, the present invention provides (1) a mobile communication system having a location management center, an exchange station, base stations, and mobile stations. The location management center registers the mobile stations in connection with location areas where the mobile stations are present. The exchange station processes calls to and from the mobile stations according to the location areas registered in the location management center and in cooperation with the base stations that control radio channels to the mobile stations. The mobile stations transmit and receive radio signals to and from the base stations through the radio channels. The location areas are each divided into paging areas. Each of the base stations or the exchange station has a storage unit for registering each mobile station in connection with one of the paging areas where the mobile station is present. The base stations have each a unit for receiving a location registration request from a mobile station and registering the mobile station in connection with a corresponding paging area in the storage unit, and a unit for determining according to the location registration request whether or not the location area registered for the mobile station must be updated, and if it must be updated, transmitting a location registration request to the location management center through the exchange station. The exchange station has a unit for receiving an incoming call to a mobile station, referring to the location management center to find the location area registered for the mobile station, transmitting an inquiry about the mobile station to the storage units according to the found location area, receiving an answer to the inquiry, and identifying one of the paging areas where the mobile station is present according to the answer, and a unit for transmitting a paging request to the base stations that belong to the identified paging area.
(2) The base stations have each a radio channel controller for controlling radio channels to the mobile stations, a wire channel controller for controlling wire channels in cooperation with the exchange station, and a storage unit for storing a mobile station identifier contained in a location registration request received from one of the mobile stations.
(3) The base stations have each a unit for transmitting a radio signal to the mobile stations, the radio signal containing a location area identifier representing one of the location areas to which the base station belongs, a paging area identifier representing one of the paging areas to which the base station belongs, and a base station identifier representing the base station.
(4) The mobile stations have each a radio controller for controlling radio channels in cooperation with the base stations and a memory for storing a location area identifier, a paging area identifier, and a base station identifier transmitted from one of the base stations.
(5) The mobile stations have each a unit for comparing a location area identifier and a paging area identifier stored therein with a location area identifier and a paging area identifier transmitted from one of the base stations, and if a difference is detected in at least one of them, transmitting to the base station a location registration request containing a mobile station identifier of its own.
(6) The location registration request further contains the location area identifier and paging area identifier stored in the memory of the mobile station. The base stations have each a unit for determining whether or not the location area identifier contained in the location registration request is equal to that of a location area to which the base station belongs, and if they are not equal to each other, transmitting a location registration request to the location management center through the exchange station.
(7) A location registration request to be transmitted from any one of the mobile stations contains a base station identifier stored in the memory of the mobile station. The base stations have each a unit for receiving a location registration request from one of the mobile stations and transmitting a deletion request to a base station whose identifier is contained in the location registration request so that the latter base station may delete therefrom a mobile station identifier representing the mobile station that has issued the location registration request.
(8) The exchange station has a unit for receiving an incoming call to one of the mobile stations, sending an inquiry about the mobile station to the storage units, receiving answers to the inquiry, and if the answers indicate a plurality of the paging areas, transmitting a paging request to the base stations that belong to the plurality of the paging areas and transmitting a deletion request to delete a mobile station identifier representing the mobile station in question to the storage units that manage the paging areas that have returned no acknowledgment to the paging request.
(9) The storage unit installed in each of the base stations or the exchange station has a unit for deleting a mobile station identifier if a mobile station represented with the mobile station identifier causes no outgoing call, incoming call, or location registration request within a predetermined period.
(10) The location management center has a unit for receiving a location registration request originated from one of the mobile stations that has just moved from one of the location areas to another and transmitting a deletion request to delete a mobile station identifier representing the mobile station in question to the storage units related to the paging areas that are in the location area from which the mobile station in question has just moved out.